Communication systems can be classified as either multipoint or point-to-point. In the latter, any given transceiver, i.e., transmitter/receiver unit, can only communicate with one other transceiver while in the former a transceiver at a central location can communicate with a plurality of other transceivers each located at a different remote location. The term "polling," when used with such networks, indicates that the transfer of data from each remote location follows a poll or request for a response by the central location to that remote location. "Multichannel" is a term which indicates that the data coupled to the central location from a particular remote location is associated with more than one information source or, if associated with the same source, is segregated and processed separately by the communication system. Examples of multichannel data in the banking industry is data associated with teller, automated teller machine and security services.
The difficulty with the implementation of a multichannel, multipoint polling network lies in the fact that each channel or application has to run independently; that is, polling of each application and the communications coupled to the central location in response thereto should proceed without disruptive interference from the other applications.
In one prior art approach, this independence is achieved by providing a separate multipoint network for each application. That is, for n applications, n multipoint networks are required. Obviously, the cost of providing this solution varies directly with n and can exceed system cost objectives even when n is 2 or 3.
Another prior art technique is to physically divide the frequency band available for remote-to-central location communications into smaller frequency bands. Each of these smaller frequency bands is then used as a data channel for one of the multipoint polling applications. This technique is known as frequency-division multiplexing (FDM). While frequency-division multiplexing provides satisfactory performance in many multichannel, multipoint network applications, such an arrangement does not readily permit dynamic bandwidth allocation for each application. In addition, the equipment required in each transceiver, i.e., transmitting and receiving unit, is essentially a multiple of that required for a single application. For example, for three applications, each associated with a different frequency band, each transceiver essentially includes three separate transmitters and receivers. This equipment requirement, coupled with the difficulties associated with bandwidth reallocation of each application over time, makes the use of FDM unsuitable for certain multichannel, multipoint network applications. Furthermore, FDM is not feasible with polling systems using digital rather than analog voiceband communication facilities.
Accordingly, it would be extremely desirable if a technique were developed for multichannel, multipoint applications that would readily permit bandwidth reallocation of each application, could be easily implemented without significant equipment costs, and could be used with digital communication facilities such as AT&T's Digital Data Service (DDS).